Conventionally, various systems for automatically controlling the running speed of a vehicle have been provided. Such control may be a control for causing a vehicle to run at a constant speed in accordance with an aimed speed, another control for causing a vehicle to run in an accelerated condition in accordance with an aimed acceleration or the like. In such controls, it may be recommended to actuate a throttle valve of an engine to adjust the output power of the engine so that the vehicle may run at an aimed speed or at an aimed acceleration.
By the way, it is advantageous in respect of control to perform such actuation of a throttle valve in automatic running control of a vehicle by means of an electrically driven throttle actuator.
Such throttle actuator, however, may possibly fail such that it stops at a certain opening because of, for example, disconnection thereof. Even in the case of such failure, it is desirable that the output power of the engine can be adjusted within an allowable range by some disposition.
The present invention has been made in view of such a subject as described above, and it is an object of the present invention to provide an engine controlling system for a vehicle which can adjust the output power of an engine within a fixed range even when a throttle actuator fails.